Technical Field
The present invention relates to radial lip seals and more particularly to unitized seals which seal between two relatively rotatable machine elements.
Brief Description of the Background Art
Radial lip seals are used in numerous rotating machine applications including wheel hubs, shaft journals and anti-friction bearings. A particular form of radial lip seal known as a unitized oil seal is preferred in certain applications because it protects the seal lips and minimizes the need for finishing the surfaces sealed by the unitized seal.
Unitized seals, which typically include an inner casing with an inner seal lip element and an outer casing with an outer seal lip element, are often installed as a unit within a bore in a wheel hub. The wheel hub is then mounted over an axle around which the seal lips form an annular sealing barrier. This mounting assembly generally involves some relative axial shifting of the inner and outer unitized seal casings and seal lip elements.
As the seal casings are axially shifted during installation, a significant axial load may be applied to the unitized seal and may result in metal-to-metal, metal-to-rubber or rubber-to-rubber contact between the inner and outer casings. Such contact can reduce seal lip life and cause an increase in torque required to rotate the unitized seal as the inner and outer casings rub against one another.
In order to prevent undesired frictional contacts between the inner and outer seal casings, prior unitized seals have used axially-extending elastomeric bumpers or nibs for axially spacing the inner seal casing from the outer seal casing. However, upon mounting the unitized seal within a housing and over a shaft, the axially-directed mounting forces will axially compress the bumpers or nibs between the inner and outer sealing casings. During the initial break-in period of the unitized seal, the nibs or bumpers on one seal casing must be worn away to eventually provide clearance or minimal contact between the bumpers and a locating surface on the other seal casing.
During the break-in period, the rubber nibs or bumpers generate significant resistance to rotation between the inner and outer casings. This resistance must be overcome by increasing the torque applied between the casings. Clearly, this increased resistance is undesirable from a vehicle efficiency viewpoint as additional energy is required to overcome the friction generated by the abrasion of the bumpers and nibs. Moreover, the heat and abraded seal particles produced by the friction can adversely affect the seal lip material and function. The abraded particles tend to migrate and collect beneath the seal lip and can result in seal leakage.
Because the nibs are initially compressed during installation, they subsequently expand axially during break-in thereby prolonging the time during which start-up torque must be increased to generate relative rotation between the shaft and bore being sealed. That is, as the nibs are worn away, they still maintain contact with the other seal casing as they axially expand to relieve their compression. This prolongs the break-in period. It has been attempted to alleviate this condition by coating the bumper surfaces with grease. However, the grease tends to migrate out of the seal assembly. Such grease migration has sometimes been misinterpreted as a seal failure, i.e., seal leakage.
A typical example of a unitized oil seal design is shown in U.S. Pat. No. 3,510,138 to Bowen which discloses in FIGS. 4 and 5 thereof the concept of providing a hard rubber surface on a unitized oil seal to minimize squealing of new seals when a seal case binds against an elastomeric portion of an oil seal. The hard rubber washer is bonded to the elastomeric body of the seal. If the seal of Bowen is assembled with excessive axial loading, the hard rubber washer will compress the elastomeric portion and result in increased torque being required to rotate the seal.
Another approach to reducing friction between adjoining seal elements is to place a washer formed of a low friction material between the seal elements to serve as a low friction thrust washer. Such an approach is taken in U.S. Pat. No. 3,761,099, to Hanson. Unfortunately, these low friction materials also abrade and migrate to the seal lips thus creating the potential for leakage problems noted above.